1. Field of the Invention
The present invention relates linear electrochemical functional elements, processes for their production, and measuring instruments using them.
2. Description of the Related Art
The primary field of application of the linear electrochemical functional element described in the following is electrochemical measurement techniques, especially with hydrogen electrodes. From German patent DE-PS 41 12 784.6 and the corresponding international application PCT/EP 92/00597, a hydrogen rod electrode with integrated hydrogen source is known. In it, a hydrogen evolution cell according to German patent DE-PS 35 32 335 is used in order to supply a hydrogen-diffusion electrode with hydrogen. Two constructions are described there which differ with respect to the active measuring electrode. The essential elements of the design are described in FIGS. 1 and 2.
In FIG. 1, the hydrogen electrode 101 is a platinized platinum wire which is positioned in the opening of a hydrogen tube 103 tapering to a point and made of glass, Plexiglas, or other material that is as impermeable to hydrogen as possible. The other end of the hydrogen tube is screwed, plugged, or glued gas-tight into the actual gas cell container 107. This preferably cylindrical gas cell container 107 holds the hydrogen evolution cell 109 according to German patent DE-PS 35 32 335. It contains zinc powder or zinc gel and caustic potash together with the so-called hydrogen evolution electrode. On the latter, a PTFE-bound catalyst film is rolled into a metal net and carries, on the side away from the zinc, a laminated-on fine-pored PTFE foil. The zinc electrode and the hydrogen evolution electrode are located in a housing which is usually made up of two metal parts insulated from each other, one of which is connected to the zinc electrode, the other to the hydrogen evolution electrode in an electron-conducting manner. The housing part containing the hydrogen evolution electrode communicates via at least one boring with the interior of the gas tube 103. The boring can be sealed by a sticker which releases the hole during operation of the cell as a result of excess pressure. The sticker may be a metal foil such as copper according to German patent application DE 195 07 658.3.
The gas cell container 107 is closed by the screwed-on or pushed-on lid 110, which may have several functions. Thus, after the closing, advisably by means of elastic spring elements (not shown), it exerts a pressure on the cell 109 so that the latter communicates by means of the ring-shaped gasket 108 through the aforementioned boring in the cell housing part with the gas tube 103. These spring elements may be the electronic contacts 112 and 113, which contact the two housing parts. The lid 110 also advisably carries a fixed or variable electrical resistance 111 in series with an on-off switch to which the contacts 112 and 113 are connected. This may be, for example, a potentiometer with an "off" position.
Instead of a lid, this electrical switching and current-regulating circuit may also be connected to the gas cell container 107.
To avoid disturbances caused by foreign gases, the metal layer from the hydrogen electrode is guided, inside the hydrogen tube if possible or embedded in its jacket, to the gas cell container where it terminates in a contact screw 106 accessible from the outside or a single-pole plug socket.
Platinum electrodes are suitable especially for use in acid media, because they resist all oxidizing acids in them. In addition, however, many other metals of the 8th Group of the Periodic Table of Elements, their alloys, or electron-conducting solid bodies metallized with them are suitable for use as long as they possess the catalytic capabilities for chemisorptive splitting of the hydrogen molecule. This is true, for example, for palladium and iridium, but also for activated carbon which is catalyzed by these metals. In this case, black, large-area coatings are characterized as especially effective. In alkaline and neutral solutions, nickel is a highly effective hydrogen catalyst, especially in the form of Raney nickel. This is a powdered material that is obtained from a nickel/aluminum alloy by extraction of the aluminum with a caustic alkali. Hydrogen electrode bodies can be produced from it by means of powder-metallurgical production processes. Such procedures are described in the book Fuel Cells by E. Justi and A. Winsel, Steiner Verlag, Wiesbaden 1962 and the patents listed in it. Electrodes suitable for this purpose, however, are also produced from a catalyst powder by intensive mixing with PTFE powder in a very high speed knife mill and rolling the powder mixture out into a metal net. Such electrodes are also advisably coated on one side with a fine-pored hydrophobic PTFE film that faces the reacting gas and keeps the three-phase boundary electrode/electrolyte/gas stable. Such electrode structures are described in European patent EP-PS 144 002 (1983). However, it may be advantageous to improve the storage capacity by using so-called hydride storage alloys in addition to the Raney nickel, as described in German patent DE-OS 37 02 138 (1987).
In FIG. 2, the hydrogen electrode 101 is such a gas diffusion electrode which is contacted with a metal ring 102 and which is affixed by the holding cap 104 to the tube 103. It may be used in any position in an electrochemical measuring cell. The coupling of the above-described hydrogen electrode with the measurement object in an electrochemical cell is determined by the geometric shape and the inflexible structure. However, it is desired to have a reference electrode which can be installed as closely as possible to the contact of an equipotential surface also in a narrowly constructed primary or battery cell.
In a polyelectrolyte (PE), one type of ion is macromolecular and polyvalent, while the counterion consists of the usual highly mobile ions. Both types of ions are hydrated and participate in water exchange in osmotic competition with their surroundings. In ion exchangers (IEs), many such macromolecular ions are connected by covalent bonds to networks that are insoluble because of their size. Therefore, they consist of fixed ions and mobile counterions which dissociate away according to the law of mass action and which can be replaced by ions of the same sign. Only these mobile counterions may contribute to electrical transport; the non-mobile fixed ions do not contribute to the conductivity. There are anion exchangers (AIEs) with mobile anions as counterions and a polyvalent cationic solid, as well as cation exchangers (CIEs) with mobile cations as counterions and a polyvalent anionic solid. Ion-exchange membranes (IEMs) are homogeneously or nonhomogeneously constructed membrane bodies which, because of their exchange action, involve only the counterions in the mass transfer by electric current and electro dialysis.
Generally, the ion exchanger excludes the penetration of ions of the same sign as that of the solid ions. In the case when ion exchangers and ion-exchange membranes are used in highly concentrated solutions, especially strong electrolytes, foreign ions of the same sign as the solid ions are carried into the solid body with the water of hydration, thereby contributing to the electrical current and electrodialytic mass transfer. However, even in such small three-dimensional elements of IEs and IEMs, the charge balance is always neutral when averaged over time. For extensive literature on IEs and IEMs refer to "Rompp Chemie Lexikon", vol. 3, published by Jurgen Falbe, Manfred Regitz (1995), Verlag G. Thieme, Stuttgart, Germany.
The swelling behavior of IEs and IEMs is determined by the water economy. It can be used to measure the concentration in aqueous solutions. A simple representation of the processes occurring here is given in the following article: A. Winsel "Concentration measurements with ion-exchange membranes," Chemie-Ing.-Techn. 44 (1972) 163-167.
If one conceives of an IE or an IEM enclosed in a solid and non-deformable housing with walls that are permeable for water vapor but are impermeable for non-water molecules, then a hydrostatic pressure gradient is formed between the inner and outer space. This is equal to the osmotic pressure difference between the IE or IEM, on the one hand, and the surrounding solution, on the other. If the osmotic pressure gradient is always directed in such a way that the enclosed IE can remove water from the environment, then the E cannot change significantly in its ion concentration nor in its water content and other characteristics.
A large number of ion exchangers and ion-exchange membranes exists. Thus the polyvalent solid may be a strong acid radical [--SO.sub.3 --].sub.n or a weak acid radical [--COO.sup.-- ].sub.n, but it may also be a strong [--N(CH.sub.3).sup.+ ].sub.n or weak base [--N(CH.sub.2).sup.+ ].sub.n. In each case, neutralized with the corresponding counterions, there are a large number of salts. They may dissociate according to the mass action laws valid in each case and conduct electric current. In suitable combination, they are buffering substances that adjust a narrow pH value. In electrochemistry in recent decades, the membrane invented by W. Grot and made of NAFION.RTM., a poly(perfluoroalkylene)sulfonic acid, has proven especially effective. NAFION.RTM. is a trademark of the Du Pont Co. The NAFION.RTM. material is available as a powder or as a solution. Ion-exchange membranes of NAFION.RTM. material are freely supporting or welded onto PTFE fabric. Membrane-like ion exchangers can be produced in various ways. This is especially simple if the powdered initial material of an IE is mixed with PTFE powder in a high-speed knife mill (reactive mixing) and a skin is formed from the cotton-like material by rolling. This IEM skin in a broad range of compositions consists of a cohesive cold-welded ion-exchange skeleton in a netlike PTFE web. An electrolytic conducting structure is formed both with and without secondary thermal treatment within the IEM body despite the hydrophobic action of the PTFE as a result of swelling. By mixing in powdered buffering salts, one can assure that the ion concentration as well as the pH will vary only slightly with the IEM in the application case.
By mixing in catalyst powders for the hydrogen reaction, biporous structures can be produced in which the IE skeleton forms the electrolyte and the catalyst skeleton the electron conductor. The gas in question may flow in the hydrophobic range of the PTFE structure. As catalyst powder, activated carbon especially is used alone or as a carrier for the mobile metal catalyst of platinum, palladium, nickel, or other metals or alloys. However, metallic catalysts may also be used as highly coarse powders such as the Raney metals. These are obtained as residual metals by dissolving an alloying component out of a powdered alloy.